This invention relates to a process for converting multifilamentary superconducting ceramic precursors into textured and densified superconducting ceramic composite articles and to the resulting articles. More particularly, it relates to a method of rolling at a reduced coefficient of friction during the breakdown stage, which increases the efficiency of conventional deformation processing techniques used in texturing superconducting composite articles and improves the physical uniformity and performance of the resulting article.
Since their discovery less than a decade ago, the superconducting ceramics have attracted wide interest, due to their ability to carry currents (below critical temperature, field and current values which are characteristic of each material) almost without resistive losses at relatively high temperatures, above about 20 Kelvin.
Composites of superconducting materials and metals are often used to obtain better mechanical and electrical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires, tapes and cables by processes such as the well-known powder-in-tube (xe2x80x9cPITxe2x80x9d) process, which includes the steps of: (a) forming a powder of superconductor precursor material; (b) filling a metal container, such as a tube, billet, or grooved sheet, with precursor powder and deformation processing one or more filled containers to provide a composite of reduced cross-section including one or more filaments of superconductor precursor material in a surrounding metal matrix; and (c) further thermomechanically processing the composite to form and sinter filament material having the desired superconducting properties. Multifilamentary composites with the desired number of filaments may be obtained by successive rebundling or cabling iterations, with additional deformation and thermomechanical processing steps as needed.
A key requirement for improving the Jc of polycrystalline superconducting ceramics is a high degree of densification and crystallographic alignment or texture of the superconducting grains. In conventional PIT processing, an initial deformation stage, commonly called the breakdown stage, is used to reduce a large diameter, low density precursor composite to a highly aspected, high density tape, cable or wire via one or more deformation drafts. Total reductions in excess of 40% during the breakdown stage are common. During the breakdown stage, the grains of the precursor phases are densified and physically aligned in relation to the direction of elongation, namely primarily such that the c-directions of the grains are aligned orthogonally to the desired current direction along the composite axis, which promotes good intergrain electrical connectivity after phase conversion. In fine multifilamentary composites, the breakdown stage also forms the basic shape of the filaments in order to promote reaction induced texture during subsequent heat treatments. Flat, evenly shaped filaments in which one dimension of the filament is no greater than about the longest dimension of the desired superconducting grains have been found to provide improved performance. Additional intermediate deformation stages, typically at low reductions, may be used after the breakdown stage to reduce the severity of reaction induced defects in the textured superconductor phases and to modulate the mosaic spread of its grains in order to further improve its texture. Between deformation stages, reaction sintering heat treatments are used to convert the oxide particle fragments of the precursor to the desired superconductor or to an intermediate phase, typically also a superconductor, to repair cracks induced by deformation, and to promote texturing by enhancing the anisotropic growth of the superconducting grains. Typical prior art processes use a breakdown stage followed by one to four intermediate stages for a total of two to five iterations, each typically involving multiple deformation drafts, although processes employing a breakdown stage with a single draft and, in one embodiment, no further iterations, have also been disclosed. See, for example, co-pending application U.S. Ser. No. 08/468,089, (US ""089) filed Jun. 6, 1995 and entitled xe2x80x9cImproved Deformation Process for Superconducting Ceramic Composite Conductorsxe2x80x9d, which is herein incorporated in its entirety by reference. The deformation sequence may be designated by the term xe2x80x9cnDSxe2x80x9d, in which xe2x80x9cDxe2x80x9d refers to the deformation step, xe2x80x9cSxe2x80x9d refers to the sintering or heating step and xe2x80x9cnxe2x80x9d refers to the number of iterations. When the steps of deforming and sintering are carried out several times, the process may be both time-consuming and expensive.
This type of uni-axial texturing has been particularly well developed for the PIT fabrication of the micaceous bismuth-strontium-calcium-copper-oxide (BSCCO) 2223 and 2212 superconducting phases (Bi2Sr2Ca2Cu3O10-X and Bi2Sr2Ca1Cu2O8-X respectively), because these oxides exhibit a modest amount of plastic deformation via the activation of the a-b plane slip systems. It is important to note that in conventional PIT processing, the deformation is applied directly to the phases of the initial precursor in the breakdown stage and to the phases of the desired superconducting ceramic or an intermediate (which typically possess either a single set of predominant slip systems as in the case of the BSCCO superconducting oxides, or no active slip system at all, as is the case with all the rare earth-containing superconducting copper oxides, the thallium-containing superconducting copper oxides and the mercury-containing superconducting copper oxides) in the remaining iterations, often called the intermediate stages.
Deformation processing of any material is complex because standard metal-working processes have both imposed stress and imposed displacement boundary conditions. In roll working, for example, standard process parameters that control these conditions include front and back tension, roll diameter, reduction, and friction coefficient. Some of these, such as tension, will most directly influence stress and others, such as reduction, will most directly influence strain. However, the influence of other process parameters, such as roll diameter and friction, is not easily predicted even in the simplest case, that of deformation of a pure metal in a system where lateral spread of the metal cannot take place. See, e.g. Avitzur, xe2x80x9cHandbook of Metal-Forming Processesxe2x80x9d, Ch. 13 for a discussion of the non-linear interactions of process parameters for roll working in such a case. Lateral spread, a significant issue in the real world, complicates matters by turning a two-dimensional system into a three-dimensional one.
The situation is even more complex in a composite material for which there are discontinuities in materials parameters at each internal interface between one material and another. Where there are significant differences in mechanical properties, such as hardness, between the two materials, geometry can be very important in determining the dominant effects. The greater the differences in the material properties, the more likely it is that localized distortions will be created at the interfaces. Moreover, material properties and processing parameters can interact in unpredictable ways. For example, deformation of a precursor powder may increase its structural integrity over time due to compaction, or decrease it due to breakup of the powder grains and/or macroscopic shear failure.
Common measures of the effectiveness of the deformation process for superconducting composites are expressed as degree of texture, core microhardness, core density, filament homogeneity and filament uniformity. High core microhardness has been associated with improved texturing and core density, but excessive microhardness has been associated with cracking. Core microhardness is a measurement of the hardness of the filament material and matrix microhardness is a measurement of the hardness of the matrix material as determined by a standard test, typically an indent test at a standard weight such as the Knoop hardness test. Core density is the density of the ceramic powder. Degree of texturing is represented by a fraction between one and zero, with one representing 100% alignment of the c-axes of the ceramic grains, such that their slip planes are parallel. Filament homogeneity is represented by a standard deviation from the average cross-sectional area of all filaments in a short transverse cross-section of the article. Filament uniformity is represented by a standard deviation from the average dimension or cross-sectional area along the length of a filament. Particularly in fine multifilament composites, low filament homogeneity and low filament uniformity have been associated with reduced current-carrying capacity (Je) possibly because of shear banding, cracking, or significant localized reductions in core density and texturing.
Common deformation techniques in nDS processes include extrusion, drawing, roll working, or pressing. While uniaxial pressing may be an effective method of both aligning the ceramic grains and densifying the filament cores (See Li et al, Physica C217, 360-366, 1993 and Korzekwa et al, Appl. Superconduct. 2(3/4), 261-270, 1994), pressing has at least one serious drawback in that it is not uniformly and continuously scalable to long lengths of superconducting material. The various forms of roll working, on the other hand, such as strip rolling, groove rolling, rod rolling, cover rolling, and turk""s heading, are well-suited for continuous processing of long lengths of superconducting material, particularly wire, tape or cable.
However, conventional roll working more typically creates undesirable defects and distortions in the composite than static forms of deformation such as uniaxial pressing. For example, rolling may sometimes induce cracks and longitudinal and transverse shear bands of the oxide filaments in a direction disruptive to current flow. Further, certain rolling conditions lead to the undesirable distortion of the oxide/metal interface, known as xe2x80x9csausagingxe2x80x9d. See, Li et al. This type of distortion is illustrated in FIG. 1, in which dark regions 10 represent oxide filaments and lighter regions 12 represent a surrounding metal matrix in longitudinal cross-section. Under certain rolling conditions, an interface 14 of the composite is distorted into a rolling, wavy conformation, resulting in alternating narrow regions 16 and wide regions 18 in the oxide filaments 10. In wide regions 18, texturing and core density is reduced. These distortions occur in both monofilamentary and multifilamentary composites, but they are particularly problematic for fine multifilamentary composites such as that shown in FIG. 1, in part because the surface to volume ratio of the filaments is so much greater.
Although cracking and shear bands may present similar appearances in multifilamentary and monofilamentary composites, their causes and solutions are not necessarily the same. The flow dynamics of monofilamentary composites and multifilamentary composites during deformation, while not fully understood, are recognized to be essentially different. For example, in contrast to the matrix metal, the powder precursor has no effective tensional strength. During deformation, the matrix must act as the element that provides structural integrity for the composite. In monofilamentary composites, for example, there is one relatively large region of powder surrounded by a matrix. There are no internal structures within the powder core that assure any level of tensional stability. As a result, the authors have seen cracks develop in monofilaments under deformation conditions where cracks are not seen in multifilaments. In comparison to multifilamentary composites, where the matrix distributes forces but there are many more internal interfaces at which discontinuities occur, monofilamentary structures provide reduced sensitivity to variations in flow uniformity between the powder core and the malleable matrix metal, but greater sensitivity to non-uniform forces and flow factors within the core itself. Although the exact source of sausaging, for example, is not known, it is believed that variations in flow compatibility both within the cores and at the core/matrix interfaces are important under different conditions.
Utsunomiya et al, Physica C, 250, 340-348, 1995 investigates the use of multiple low reduction drafts, increased roll diameter and lubrication with mineral oil in a post-sintering intermediate rolling sequence to obtain improved Je performance in monofilamentary composites. Utsunomiya et al teaches that lubrication during the intermediate stage is associated with slight increases in texturing (but none in grain alignment). Utsunomiya et al does not consider the impact of lubrication, total deformation, or number of deformation drafts during the breakdown stage, nor does it address the issue of reducing deformation defects at all.
Cracking, formation of shear bands and sausaging may be reduced in a rolling process by the use of multiple low reduction stages. See, Korzekwa et al. for a discussion of this approach to reducing defects in monofilamentary composites. While this approach may reduce sausaging, it has not been shown to eliminate it entirely. Additionally, low reduction drafts exert only a small penetration force on the composite article, so core density remains very low. EPO 0 504 908 by Sumitomo Electric Industries (EP ""908) identifies this density problem and describes the preparation of monofilament superconducting oxide wire by multiple reduction stages using high friction rolls, preferably using increasing frictional force during each successive stage. EP ""908 reports increases in core density and Je caused by the increase in friction between the rolls and the composite.
In multifilamentary composites, certain rolling conditions (See Li, et al) also lead to undesirable heterogeneities among filaments in core dimensions, hardness, density and texture. This is illustrated in FIG. 2, in which dark regions 21 represent oxide filaments and lighter regions 22 represent a surrounding metal matrix in short transverse cross-section. Barrel zones 24, center dead zones 26 and extension zone 28 are undifferentiated regions of composite 20 prior to deformation, but will be subject to different stress and flow conditions during deformation which create variations in filament characteristics thereafter. Barrel zones 24 are areas where lower compressive pressure is exerted during roll working. Center dead zones 26 are areas of limited material flow due to surface friction. Under certain rolling conditions, filaments in the barrel zones 24 and center dead zones 26 at the edges of composite 20 will have lower uniformity and poorer texture than filaments in extension zone 28 of the composite 20, while the filaments in extension zone 28 will exhibit undesirable short transverse shear bands. In addition, filaments in the barrel zones 24 at the edges of composite 20 will have lower core hardness and density, as well as significantly larger and less uniform cross-sections than filaments in the other areas.
Cracking, formation of shear bands and sausaging in multifilamentary composites may also be reduced by the use of a high reduction roll working draft during the breakdown stage, as described in US ""089. While this approach reduces sausaging, formation of longitudinal shear bands, and cracking, it creates multi-modal transverse shearing, a form of short transverse shearing defect which is unique to multifilamentary composites. Moreover, while using a high reduction breakdown draft reduces filament heterogeneity in the dead zones when compared to the results of multiple low reduction drafts, significant filament heterogeneity remains and further improvements are desirable.
The approach described in US ""809 also has an extremely sensitive process response surface, with small variations from optimum processing parameters creating high dimensional variations and large decreases in Je. This creates difficulties for large scale manufacturing, as extremely precise control over deformation conditions is hard to maintain over extremely long lengths of wire, cable or tape. Low dimensional variations are a key product specification for many applications, such as coils.
Therefore, it is an object of this invention to provide a method for forming a high performance multifilamentary superconducting article having low dimensional variation over long lengths.
It is a further object of this invention to provide a method for forming a multifilamentary article having fine and uniform superconducting ceramic filaments without multimodal transverse shearing, cracking, shear bands or sausaging.
It is a further object of this invention to provide a method for forming a multifilamentary article having reduced dead zones and improved filament homogeneity in the dead zones.
It is a further object of this invention to provide a method for forming a precursor composite having greater filament homogeneity and uniformity across deformation zones.
It is a further object of this invention to provide a method for forming a precursor composite having lower differential impact on the microhardness of the matrix and the filaments.
It is a further object of this invention to provide a method for deforming a multifilamentary superconducting article which is effective despite variations in processing conditions typical of large-scale manufacturing operations, and which may be used to optimize precursor density and uniformity in a limited number of processing steps.
The inventors have found that a reduced coefficient of friction, preferably created by ideal lubrication conditions, may advantageously be employed in the breakdown stage of deformation processing, particularly in combination with one or more high reduction drafts, to improve composite homogeneity and significantly increase the range of deformation conditions over which dimensional tolerances and Je may be optimized. Precursor composites made by this method exhibit reduced microhardness variability and fewer and less serious transverse filament defects than composites made by prior art methods.
In one aspect, the invention is a method for manufacturing a multifilamentary superconducting ceramic composite article comprising the steps of: first, providing a precursor article comprising a plurality of filaments extending along the length of the article and containing precursors to a desired superconducting ceramic, and a metal matrix substantially surrounding the filament; next, roll working the precursor article during a breakdown stage at a predetermined pressure and a coefficient of friction less than about 0.2 during each roll working draft and, then, sintering the rolled article to obtain the desired superconducting ceramic. The coefficient of friction is preferably less than about 0.01 and most preferably less than about 0.001 during each roll working draft.
In another aspect, the invention is a method for manufacturing a multifilamentary superconducting ceramic composite article comprising the steps of: first, providing a precursor article comprising a plurality of filaments extending along the length of the article and containing precursors to a desired superconducting ceramic, and a metal matrix substantially surrounding each filament; next, roll working the precursor article during a breakdown stage at a predetermined pressure while using a lubricant between the article and one or more rolls during at least one roll working draft; and, then, sintering the rolled article to obtain the desired superconducting ceramic. It is most preferred that the lubricant be selected to create ideal lubrication conditions, that is, to maintain the coefficient of friction between the article and one or more rolls at less than about 0.001 throughout each roll working draft. However, boundary lubrication conditions, in which the coefficient of friction ranges from about 0.01 to about 0.001, and even marginal lubrication conditions, in which the coefficient of friction ranges from about 0.2 to about 0.01 and some contact between the article and the rolls does occur, may also be effective.
These methods are applicable to any deformation process which employs roll working during one or more drafts of the breakdown stage, but are particularly effective for processes in which there is at least one high reduction roll working draft during the breakdown stage. The pressure during each roll working draft is preferably greater than about 1 MPa, and most preferably greater than 10 MPa. It is preferred that the total reduction achieved in the breakdown stage be on the order of 40% to 95%. Most preferably, a single high reduction roll working draft is used during the breakdown stage and no further reduction of the article in excess of about 10% (preferably less than 5%) occurs after the high reduction roll working draft and before the first sintering operation. By xe2x80x9cno further reduction of the article in excess of about 10% occurs after the high reduction roll working draftxe2x80x9d is meant that no other deformation processing occurs during the breakdown stage after the high reduction roll working draft and before the sintering step. However, it is contemplated that the breakdown stage may be the initial step in an nDS process where n is greater than 1, so that additional deformation, including lubricated or unlubricated roll working, and sintering steps may occur in other DS steps and still be within the scope of this invention. In addition, other processing operations may be contemplated at this stage and, of course, later stages, of the process including an ODS (oxide dispersion strengthening) treatment, anneals, shaping, machining, cabling, coiling, winding or other chemical or mechanical processing.
It is preferred that the deformation process be an nDS process, where n is no greater than 5. In one aspect, it is preferred that n be 1, i.e., that the breakdown stage be the last stage in which significant deformation of the article occurs, although minor deformation incidental to other operations such as finishing, forming or cabling may of course occur thereafter. In another aspect, it is preferred that n be equal to 2 or 3, that the total reduction achieved during the breakdown stage be on the order of 40% to 95%, and that the total reduction achieved in the intermediate stages be on the order of 2% to 25%.
By xe2x80x9croll workingxe2x80x9d, as that term is used herein, is meant the process of passing a precursor article such as a round wire or rectangular tape through a constrained gap of one or more, i.e., a pair or a four-way turks head arrangement, of rollers, so that deformation and reduction in at least one lateral dimension of the article results. By xe2x80x9cdraftxe2x80x9d as that term is used herein, is meant the reduction in thickness of an elongated superconducting article in a single deformation operation. A xe2x80x9cstagexe2x80x9d as that term is used herein, comprises one or more successive drafts, with or without roll working but without intermediate sintering operations.
By xe2x80x9csinteringxe2x80x9d, as that term is used herein, is meant heat treatment of the composite precursor article under conditions sufficient to convert a portion of the precursor into the desired superconducting ceramic. Where the desired superconducting ceramic is BSCCO 2223, sintering preferably includes heating at a first temperature in the range of 800-850xc2x0 C., heating at a second temperature in the range of 700-840xc2x0 C. and heating at a third temperature in the range of 600-800xc2x0 C. Sintering includes heating at an oxygen partial pressure of 0.0001 to 100 atm. By xe2x80x9cannealxe2x80x9d is meant a heat treatment under conditions which create no substantial phase changes in the desired superconducting oxide or its precursor.
The invention provides multifilamentary superconducting composite articles and rolled precursor articles with improved uniformity in filament microhardness, filament shape and aspect ratio, and filament/matrix microhardness. The rolled precursor article and resulting superconducting ceramic composite article are preferably elongated forms such as wires, tapes, cables, or current leads, and may comprise twisted or untwisted filaments. By xe2x80x9caspect ratio""xe2x80x9d is meant the ratio of the width to the height of the filament or article, as measured in transverse cross-section.
In one aspect, the invention provides a rolled precursor article comprising a plurality of filaments extending along the length of the article and containing precursors to a desired superconducting ceramic, and a metal matrix substantially surrounding each filament, the precursor article having a difference between the average microhardness of the filaments and the average microhardness of the matrix of less than about 40, as measured by the Knoop hardness number with a load of 10 grams. In another aspect, the invention provides a rolled precursor article comprising a plurality of filaments extending along the length of the article and containing precursors to a desired superconducting ceramic, and a metal matrix substantially surrounding each filament, in which all filaments have a microhardness between about 100 and 160, as measured by the Knoop hardness number with a load of 10 grams. In another aspect, the invention provides a rolled precursor article comprising a plurality of filaments extending along the length of the article and containing precursors to a desired superconducting ceramic, and a metal matrix substantially surrounding each filament, in which the difference in average filament microhardness among deformation regions is less than about 40, as measured by the Knoop hardness number with a load of 10 grams.
In another aspect, the invention provides a rolled precursor article comprising a plurality of filaments extending along the length of the article and containing precursors to a desired superconducting ceramic, and a metal matrix substantially surrounding each filament, in which the aspect ratio of each filament is at least about 3. In another aspect, the invention provides a rolled precursor article comprising a plurality of filaments extending along the length of the article and containing precursors to a desired superconducting ceramic, and a metal matrix substantially surrounding each filament, in which the aspect ratio of each filament is at least about 15% of the aspect ratio of the article, as measured in transverse cross-section.
In another aspect, the invention provides a multifilamentary superconducting ceramic composite article which comprises a plurality of filaments extending along the length of the article and containing a desired superconducting ceramic, and a metal matrix substantially surrounding each filament, in which the aspect ratio of each filament is at least about 3. In another aspect, the invention provides a multifilamentary superconducting ceramic composite article which comprises a plurality of filaments extending along the length of the article and containing a desired superconducting ceramic, and a metal matrix substantially surrounding each filament, in which the aspect ratio of each filament is at least about 15% of the aspect ratio of the article, as measured in transverse cross-section.
The invention may be practiced with the precursors of any desired superconducting ceramic which requires texturing and may be entirely or partially textured by deformation. The compounds are preferably precursors of superconducting oxides, and particularly of the bismuth, rare earth, thallium or mercury families of superconducting copper oxides. Precursors of the bismuth family, and particularly its 2223 phase, are most preferred. By xe2x80x9cprecursorxe2x80x9d is meant any material that can be converted to a desired superconducting ceramic upon application of a suitable heat treatment. Where a superconducting oxide is the desired superconducting ceramic, for example, precursors may include any combination of elements, metal salts, oxides, suboxides, oxide superconductors which are intermediate to the desired oxide superconductor, or other compounds which, when reacted in the presence of oxygen in the stability field of a desired oxide superconductor, produces that superconductor.
In preferred embodiments, the metal matrix includes a noble metal. By xe2x80x9cnoble metalxe2x80x9d is meant a metal whose reaction products are thermodynamically unstable under the reaction conditions employed relative to the desired superconducting ceramic, or which does not react with the superconducting ceramic or its precursors under the conditions of manufacture of the composite. The noble metal may be a metal different from the metallic elements of the desired superconducting ceramic, such as silver, oxygen dispersion strengthened (ODS) silver, or a silver/gold alloy, but it may also be a stoichiometric excess of one of the metallic elements of the desired superconducting ceramic, such as copper. Silver (Ag), ODS silver, and silver alloys are the most preferred noble metals.